While optically variable devices have a number of uses, e.g., packaging, decorative applications, etc., these devices have found significant use in various security devices. Security devices are used to protect a variety of valuable objects such as currencies, financial documents, travel and identification papers, and branded products. The primary function of these devices is to prevent counterfeiting, and thus these devices are designed to make the replication of the article to which they are attached difficult and/or expensive.
Security devices are classified by the manner in which they are authenticated. First level or firstline inspection refers to features of the security device that are checked directly through a user's senses, typically the user's tactile or visual senses. Common examples of such features include color-shifting inks, holograms and intaglio print. Since these types of security devices do not rely upon the use of equipment for detection, they are commonly known as overt security devices.
Security devices that are designed to be shielded or hidden from direct sensory detection by the user of the article to which the device is attached are commonly known as covert security devices. If only simple tools or equipment, e.g., a magnifying glass, ultraviolet light or a polarizing sheet, are needed for detection, then the security device is classified as a second-line or second-level device. Various industry personnel, such as bank tellers, cashiers and first-line government workers who do not require extensive training, typically perform these types of inspections. Second-level features include ultraviolet fluorescent inks, polarized images, bar codes and magnetic inks. Third-line or third-level security devices are those that require forensic specialists and/or sophisticated laboratory equipment to detect.
OVDs are a particular class of overt security devices. These devices (or technologies) are constructions of multiple materials that change color or appearance depending upon the angle from which they are viewed. Devices that change their appearance due to changes in the tilt angle are more common than those devices that change color or appearance upon rotation. Most OVDs are appropriate for first-line inspection with the human eye. Examples of such OVDs include metameric colors, three-dimensional tilt images, parallax images, lenitcular images, holograms, kinegrams and zero-order devices.
Currently holograms are the dominant OVD used for the protection of consumer goods. They appear on credit cards, various documents and currency, and an assortment of branded products. Effective as they are, holograms have a number of drawbacks as security devices.
One such drawback is that diffuse lighting reduces the brilliance of holographic images and as such, they often appear dim and blurry under fluorescent light or an overcast sky. Another drawback is that dark colors, especially black, are excluded from the color gamut. Yet another drawback, and one more significant than most others, is their susceptibility to counterfeiting.
Embossed holograms can be stripped from the substrate to which they are attached to permit a tooling replication. Holograms typically comprise at least one polymer layer over-riding a metal layer. The polymer layer or layers can be stripped from the hologram to expose the metal layer, and a contact process can then replicate the metal layer. Photopolymer volumetric holograms cannot be counterfeited physically, but they can be copied optically as can most planar or multi-planar holograms.
Buried grating microstructures offer a solution to this hologram replication problem. These microstructures comprise a three-dimensional structure of grating lines with a high index of refraction buried or embedded in a material with a low index of refraction. When the length of the grating lines are on the order of or shorter than the wavelength of light that penetrates these lines, these microstructures exhibit a peculiar specular or zero-order reflection that is different for light polarized parallel to the grating lines than for light polarized perpendicular to the grating lines. The result is that the color observed in reflection or transmission changes when the surface of the object in which the grating lines are embedded is rotated or tilted. The color change observed upon rotation is a unique feature of zero-order gratings; for many OVDs color change is observed only when the structure is tilted, not rotated. The reflected colors are bright and clear even under diffuse light. The buried grating is an integral part of the structure and as such, it cannot be separated from the structure. Moreover, it is resistant to both mechanical and optical copying techniques (the latter because the optical behavior of the device is determined by the physical variations in the refractive index of the device, and these variations are not susceptible to optical copying).
The components of known zero-order grating microstructures are a low refractive index polymer and a high refractive index ceramic material. See, for example, van Renesse, R. L., Optical Document Security, 2nd Ed., pp. 267-286, Artech House Publishers, Boston (1997). The ceramic material is typically slope-evaporated onto the polymer under vacuum. This is a relatively expensive and slow process and, at least in part, it is the reason for the presently limited uses of these microstructures in commerce.